Process Description for the Manufacture of Oxygen and Nitrogen from
Five Step Pocess For The Manufacture
Of Oxygen And Nitrogen
1. AIR COMPRESSION
Air is drawn from atmosphere through Suction Air Filter. (1).Air is drawn
from atmosphere through Suction Air Filter to prevent dust from getting into
The air is then compressed in a three stage Air Compressor (2) with after
cooler to a maximum pressure of 55 Kgs./Cm2 for plant starting conditions
and a pressure of 40 - 45 Kgs./Cm2 for best results normal running
conditions. Depending on ambient conditions and good operations the
operating pressure of the Air Separations Unit is brought down to 40 Kg/Cm2
as per past experience. Air Compressor has inter-coolers between stages and
an After-Cooler after 3rd stage. For further details on Air Compressor,
please refer to Air Compressor Manual supplied by the Air Compressor
manufacturer. The air compressor should be maintained properly in good
condition as it is the main source of air supply to the plant.
2. AIR -PURIFICATION
The air then enters into cascade an Evaporating Cooler (5) where it gets
cooled to about 20 Deg.C. This unit is optional. The cooler is a cubical
vessel where, there is pipe coil and is inter connected. The coils are half
submerged in water in the vessel Dry Nitrogen will be bubbled through this
water to become wet gas. As the water vaporizes, it requires latent heat
which is absorbed from water itself. So, water gets cooled. Thus, air inside
the pipe coil will get cooled. Compressed air, cooled in evaporation cooler
will enter into a Moisture Separator (4 & 8).
Moisture condensed as water will be separated and drained once in an hour.
It is important to drain moisture from the bottom of the Oil Absorber (9) at
regular intervals and also change the Alumina every 6 to 12 months. After
this the air will pass through an additional cooler called Chilling Unit
After this the air will pass through Oil Adsorber. (9) Packed with Alumina
balls. Here the Oil Vapor carried over from Air Compressor will be removed.
If this oil vapor is not removed sufficiently, due to spent carbon or due to
high temperature of process air, the oil vapor will damage the Molecular
Sieves. To obtain a long life of Molecular Sieve ensure the Alumina is well
The air then enters one of the Molecular Sieve vessels (11). The moisture
and carbon dioxide in the air will be removed in this drier. If they are not
removed before entry to Cold Box, they will form Ice and dry Ice which will
choke the Heat Exchanger Tubes and other equipments. There are two driers.
One will be (on line with the process air) in operation for around 10 hours
and the other will be under regeneration. Regeneration is done by heating
and cooling with not-going Nitrogen. An electric regeneration gas heater
(12) is used for regeneration. For further details, refer separate chapter
on Molecular Sieve Driers.
The dry air is again filtered in a Dust Filter (13) before entry to Cold
Box to avoid any dust entry to Cold Box. In some plants the air is further
cooled through special coils provided in the Chilling Unit Tank (6), which
is called an equalizing coil as it equalizes the temperature after the
Molecular Sieve drier before Air enters the Cold Box.
3. COOLING OF AIR
The compressed air, cooled to about 15 to 20 Deg.C free of moisture and
carbon dioxide will enter the Cold Box (15). It initially passes through a
Heat Exchanger No.1 (16); the incoming air will be cooled by the outgoing
Oxygen and Nitrogen. The air will be cooled to around -100 Deg.C. In this
Heat Exchanger. This can be single or divided two parts in series.
The air will then be into two streams. The main air stream will enter
Expansion Engine (14) at 40 - 45 Kgs./Cm2 and will be expanded to 5 Kgs./Cm2
and -150 to 160 Deg.C the rest of the air will pass through Heat Exchanger
No. 2 (17) to be cooled to about -160 Deg.C. by the outgoing Oxygen and
Nitrogen. This air will then be expanded by an Expansion Valve V3 to form
liquid air. Both the air streams will now enter bottom portion of the Lower
Column (19). Operating pressure of the column is around 45 kg/cm2 under
normal operating conditions.
As the air enters the Lower Column, after the Expansion Engine and after
Expansion Engine valve V3, a part of this air condenses into liquid and
falls at the bottom of the column. This liquid is about 40% Oxygen and 60%
Nitrogen and is usually called the "Rich Liquid" and as Nitrogen
is more volatile it rises to top of the lower column where it gets cold from
the condenser and become liquefied. This liquid nearly free of oxygen
collected in the (Pockets in the condenser) trap. As this liquid poor in
oxygen is called poor liquid.
4. AIR SEPARATION
Final separation of the two fractions is achieved in the upper column. Both
the poor liquid are carried into the upper column by two Expansion Valves
and the pressure drops from 4.5/5.0 Kgs. /Cm2 in the lower column to 0.5
Kgs. /Cm2 in the upper column. The rich liquid enters the middle of the
Upper column and as it flows down, Nitrogen evaporates and Oxygen continues
as liquid. The Liquid Nitrogen (Poor Liquid) enters the top of the column
and as it is flows down the column, it comes in contact with any evaporating
Oxygen and condenses the same into liquid, while the Nitrogen itself becomes
a Gas as it is more volatile. This process takes place in each Gas as it is
more volatile. This process takes place in each tray. The entire gaseous
Nitrogen is piped out from the top of the column through Heat Exchangers.
Similarly the Liquid Oxygen at the bottom of the column is carried away to a
Liquid Oxygen Pump from which it is compressed and again passed through the
Heat Exchangers into the Gas Cylinders. As the Liquid Oxygen travels through
the Heat Exchangers, it evaporates into gaseous oxygen filling the cylinder
with gas and giving up its cold to the incoming air
Generally the purity of Oxygen will be 99.5% and Nitrogen about 96%, when
the plant is operated exclusively for oxygen production.
The Plant operation should be such that it is not too cold or too warm. If
the cold box is too cold, the Nitrogen will condense into Liquid Oxygen and
the Oxygen Purity will fall.
If the plant is too warm oxygen will evaporate with the Nitrogen and the
quantity of Oxygen produced will go down substantially and the waste
nitrogen will carry more and more oxygen. To obtain optimum result of the
plant, therefore check the purity of the waste Nitrogen which should not
fall below 96%.
When the plant works continuously for a few months, it tends to accumulate
Carbon Dioxide and moisture in its internal parts. These are to be removed
once in about four months. For details, refer chapter on Defrosting of
Similarly, the L.O. Pump alone can be defrosted in case of trouble in
pumping (Refer L.O. Pump chapter).
It is advised to give Carbon Tetra Chloride wash to the Cold Box equipments
once in a year to ensure protection against Hydro Carbon contamination. But
when starting during commissioning CTC wash is a must.
Before starting plant, it is generally defrosted and blown out. That the
cooling/starting is done which will take about 7 to 8 hours. When the plant
is stopped for short intervals, the plant need not be defrosted, but all the
cold line valves are to be closed to prevent outside moisture from entering
the Cold Box.
5.FILLING OF LIQUID /GAS OXYGEN
THE FINAL STEP IS THE FILLING/ COMPRESSION OF LIQUID OXYGEN PUMP The liquid
oxygen is withdrawn from the condenser and is flows to the liquid oxygen
pump where it is compressed automatically into compressed oxygen. For
filling oxygen cylinders the oxygen will go to the oxygen filling manifold
Industrial Plants Manufacturing Co.(P) Ltd.
Mr. Sanjiv Agrawal
A-104/2, Okhla Industrial Area, Phase - II
New Delhi - 110 020, Delhi, India
Universal Industrial Plants Manufacturing Co.(P) Ltd.
Developed and Managed by IndiaMART